Investigating Causes Of Crystal Oscillator Failure To Start

Locating the Crystal Oscillator on the Circuit Board

The crystal oscillator is a critical component in many electronic devices that provides a clock signal for timing and synchronization. However, crystal oscillators can sometimes fail to start up properly, preventing the system from booting or operating correctly.

The first step in diagnosing a crystal oscillator failure to start is to locate the component on the circuit board. Crystal oscillators come in a standard two-pin can or four-pin surface mount package. Using the board schematic as a reference, visually inspect the board to identify the crystal oscillator component.

Check nearby components and traces for any signs of damage, overheating, or discoloration which could impact the crystal oscillator’s operation. Carefully probe the crystal oscillator pins to measure if the expected supply voltage is present. A missing or incorrect voltage could prevent startup. If issues are found, troubleshoot the power supply circuitry before the oscillator.

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Testing the Crystal Oscillator Voltage

After locating the crystal oscillator, use a multimeter to check the DC bias voltage applied to the component. Consult the datasheet to determine the required startup voltage, typically 1-5V. Carefully probe the voltage at the oscillator pins while power is applied.

If the expected voltage is not present, there could be an issue with the bias voltage supply circuit. Check any bias resistors, filtering capacitors, regulator ICs, and upstream components in the circuit for faults. Insufficient voltage prevents the piezoelectric crystal from vibrating at its specified frequency.

If the bias voltage checks out, measure the AC amplitude of the clock output signal. An extremely low amplitude could indicate the oscillator attempted starting up but failed. Determine if the startup failure is due to excessive load capacitance or resistance improperly damping the crystal.

Checking for Physical Damage to the Crystal

Carefully inspect the surface of the crystal oscillator component for any signs of physical damage using a magnifying glass. Look for cracks in the epoxy resin case or broken/bent pins which could impact electrical connections or environment protection.

Shock, vibration, mishandling during assembly, or excessive heating during soldering can cause physical damage. Crystal oscillators rely on the precise physical dimensions and properties of the internal piezoelectric crystal resonator. Any damage or distortion can prevent startup.

If damage is present, the oscillator component must be replaced. However, further root cause analysis should still be performed to check why mechanical stress occurred to prevent recurrence in other units. Implement vibration damping, circuit board supports, or production process changes as needed.

Verifying the Load Capacitance

The crystal oscillator relies on a specified tank circuit with load capacitance to determine the resonant frequency. Insufficient or excessive load capacitance connected to the crystal can prevent reliable startup. Use a capacitor meter or impedance analyzer to measure the actual load capacitance.

Compare to the datasheet recommendations for the crystal and stray board capacitance. Trim the discrete load capacitors as needed to achieve the target value. Load capacitance tolerance directly impacts frequency tolerance and startup reliability per the oscillator barkhausen criteria.

Keep load capacitance connections short and use RF-rated bypass/de-coupling capacitors on the oscillator power pins. This prevents parasitic inductance or noise injection from inadvertently altering the capacitive load seen by the crystal.

Measuring the Equivalent Series Resistance

The oscillator crystal exhibits an extremely high but finite impedance with an equivalent series resistance or ESR determined by its mechanical construction. Using an LCR meter, carefully measure the ESR at the specified resonant frequency looking into the crystal terminals.

Typical ESR values range from 10-100 ohms. Excessive crystal ESR due to material impurities or physical defects reduces the Barkhausen signal amplitude for startup. If crystal ESR is too high, replace the defective component to restore design margins.

Monitor ESR distributions among production crystal lots and specify appropriate vendor limits. Statistical process control tracking of ESR helps gauge manufacturing capability and detect deterioration over time.

Replacing a Damaged Crystal Oscillator

If previous troubleshooting has determined the crystal oscillator component itself is damaged, carefully de-solder and replace the faulty component. Use a low-wattage soldering iron and vacuuming desoldering tool to prevent circuit board damage.

Visually inspect solder joints, clean up any excess flux, and check for shorts or opens before applying power. Verify the new oscillator starts up and achieves the specified frequency using a frequency counter connected to the oscillator output pin.

Replacing the oscillator should restore operation. However further analysis using an oscilloscope may be warranted to check for noise sources coupling into the oscillator circuit and impacting performance.

Design Tips to Prevent Issues

Several best practice design guidelines can improve crystal oscillator startup reliability and prevent field failures:

Select oscillator components from established, high-quality vendors with rigorous quality control over circuit trim and piezoelectric blanks.
Screen components for frequency tolerance, ESR distribution, and startup voltage thresholds.
Include modest overhead in bias supply voltages and test startup margins across voltage, temperate, and tolerance extremes.
Carefully model and measure timing delays from initial power-on until clock stabilization.
Include diagnostic access to oscillator waveforms and frequency counters to facilitate troubleshooting.

Paying close attention to proper crystal oscillator selection and circuit design can minimize issues with startup and stability in fielded systems.